[nailgun] Root size calculation blocks deployment on non-first drive in CentOS

[Dmitry Borodaenko]

When deploying CentOS on a second drive that is significantly smaller than the first drive, deployment is blocked because minimum root partition size gets set to a size larger than the second drive. There's more than one problem with the way root size is calculated in _calc_root_size():

1) Hardcoded assumption that the size of the _first_ drive in the system is relevant to determining the OS VG size (not true on CentOS where the OS volume group can go on any drive or even span multiple drives).

2) Hardcoded assumption that drives smaller than 50GB are not going to be used to install OS (not true for customers who want to put OS on a small SSD).

3) Confusion between the _default_ root size and _minimum_ root size. Minimum root size should still be 10GB (or whatever amount of disk space is required to install _all_ Mirantis OpenStack components with all options enabled). Default size may be larger than that but should not be used to validate disk sizes before deployment.

This problem was brought up in a code review but didn't get addressed before the change was merged:
https://review.openstack.org/#/c/56322/6/nailgun/nailgun/volumes/manager.py

[Andrew Woodward (xarses)]

Please explain Won't Fix status